The present invention relates to a focus control device of an optical disk apparatus, and more particularly relates to a focus control device having an automatic setting function of an optimal offset signal of an optical magnetic disk.
In order to read and write data precisely in an optical magnetic disk apparatus, it is necessary to reduce the focal error into the range of focal depth of an objective lens. Therefore, focus servo control is conducted in the apparatus. One typical focal error detecting system used for an optical disk apparatus is the astigmatic method in which cylindrical lenses causing astigmatism are utilized. According to this method, the focus error signal is represented by an S-shaped curve shown in FIG. 3 with regard to the focal error in the servo block range. Therefore, in an ideal condition, the voltage of the focus error signal becomes 0V (which is the same as ground) at a correctly focused point. When the device is designed, the target control value is 0V (ground), and feedback control is conducted so that the level of the focus error signal can converge to the ground level.
However, in a practical device, depending upon the error caused in the manufacturing process of the optical disk, the characteristic error of the optical system, the characteristic error of the circuit, and changes of the characteristics caused by changes of the environment, the actual correctly focused point (referred to hereon as an actual focus point) is shifted from the ground, and offset V.sub.off exists in the position shown in FIG. 3. Therefore, in order to make the servo control target value (the convergence point) coincide with the actual focus point, the following method has been conventionally taken in the manufacturing process: a semi-fixed resistance is adjusted in order to impress the offset voltage upon the control section of the focus drive mechanism.
The conventional method in which a predetermined offset signal is previously given in the manufacturing process, cannot follow the change of the optical system caused by environmental change or deterioration with age, and further the conventional method cannot follow the very small change of the actual focus point which is caused by the offset of the circuit. Accordingly, when high accuracy is required, reliability is lowered.
According to the results of the investigation conducted by the inventors, the conventional adjusting method has the following problem in the case where the disk is changed: especially when a seeking operation is conducted, in other words, when a reading-out beam is rapidly moved to the target track, the accuracy of the track position detection signal is deteriorated, so that the beam cannot be correctly stopped at the target track; and further the actual focus point in the case of a playback/write-in operation is different from that in the case of a seeking operation.